The Synthesis of Nitroferrocene
Copyright 2001 Russell Gulliver
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Front cover designed by Russell Gulliver
3D-Computer Rendered Fairy by Cheyenne
Other Information
Typeset: Times New Roman 12pt/13pt
ISBN: 978-1-4661-5598-5
WARNING
Do not carry out this work unless you are a University trained chemist or above due to the potential dangers of the project.
Table of Contents
Results and Discussion: Infra Red Spectrum
Results and Discussion: Problems and Difficulties
The aim of this project is to produce mononitroferrocene and to try to outline and identify any problems with the synthesis of the compound while increasing the yield. The full list for aims and objectives are outlined below:
Aims and Objectives:
Synthesise and identify any problems in producing mononitroferrocene
To try and increase the yield of mononitroferrocene.
Try to produce dinitroferrocene and reduce to the diaminoferrocene.
To co-ordinate the diaminoferrocene onto a metal centre as a ligand and study the effects/properties.
Ferrocene, Cp2Fe is an organometallic or metallocene compound. In the compound the number of electrons possessed by the metal adds up to eighteen, ten electrons from the cyclopentadienyl ligands and eight from the iron atom itself, Thus giving the iron atom a stable configuration.1 The bonding interactions in ferrocene are between the cyclopentadienyl anti-bonding orbitals and the iron's d-orbitals, mainly the e1-type orbitals (dxz , dyz ) which interact with the e1-type ring p orbitals.The a1 type orbitals, which have the lowest energy, also have favourable overlap with the iron d-orbitals. As stated above the e1g set of degenerate orbitals overlaps with the dxz and dyz orbitals, thus forms a strong pi-bond. The e1u interaction between the iron px and py also gives some stabilisation. The dx2- dy2 and dxy can overlap with the e2g orbitals of the ligand, however this bonding interaction is very weak and in this instance can be classed as non-bonding.
In the case of ferrocene a principal axis of symmetry can be chosen to pass through the metal centre and intersect the ring plane perpendicularly at the ring centre i.e. [Cp2Fe] is a pentagonal bipyramid, symmetry D5d or D5h.1
The discussion about bonding does not depend critically on the preferred conformation of ferrocene, whether staggered (D5d) is favoured over eclipsed (D5h) or vice versa, the energy barrier for rotation is about 8-20 kJ mol-1.1 Even though the eclipsed phase is more stable, in condensed phases such as crystals, where the intermolecular energies of the same or greater magnitude both forms of the configuration maybe found. However in 1H-NMR the cyclopentadienyl hydrogens appear as a singlet from a shift range of 4.0 upto 5.5 ppm, since the energy of rotation is quite low and the molecule possesses a low symmetry, the ring protons appear equivalent on the NMR timescale.
However cyclopentadienyl compounds are the most important carbocyclic p complexes and the [C5H5]- ligand or substituted derivatives such as [Cp2Fe] are widely used.1 The formation of these compounds depends on the fact that cyclopentadiene is a weak acid (pKa ~ 20) and with a strong base gives the salt of the symmetrical cyclopentadienide ion.
The synthesis of nitroferrocene is in two steps, the first step is to produce the 1,1'-dilithioferrocene-TMEDA and the second step is to react the 1,1'-dilithioferrocene-TMEDA with isopropyl nitrate to produce the product, which is nitro ferrocene.
In the first step ferrocene is converted into 1,1'-dilithioferrocene-TMEDA, the reaction relies on the fact that an organic compound containing a relatively acidic hydrogen reacts with an organolithium reagent to undergo a hydrogen-lithium interchange.2 However using a organolithium reagents does have the drawback that in the presence of tetrahydrofuran (THF) they are relatively unstable. This can be overcome by using the organolithium reagent in a large excess, but this also has associated problems, in that the reagent can react further with the newly formed organolithium reagent. Essentially this can be overcome by a method devised by Eberhardt and Butte and by Langer.2 They reported the use of N,N,N',N'-tetraethylenediamine (TMEDA) or 1,4-diazabicyclo[2,2,2]octane (DABCO) which form stable co-ordination complexes with various organolithium reagents, and that these complexes are considerably more reactive in the metalation reaction than the organolithium reagents alone.
Purification and isolation of the 1,1-dilithioferrocene-TMEDA was not carried out as the compound is extremely air sensitive and attempts to isolate and purify the product before continuing to the next step would be unfeasible. Hence isolation and purification was considered to be beyond the scope of this project.
The reason for using the method where 1,1-dilithioferrocene-TMEDA is reacted with isopropylnitrate, instead of using the nitration method used for benzene
is that even though this method would work on cyclopentadiene alone, the nitric acid and sulphuric acid mix would oxidise the iron centre in ferrocene, from a plus two oxidation state to the plus three oxidation state of the deep blue ferricenium cation [Cp2Fe]+.3 Hence rendering ferrocene inactive for reaction with the nitronium ion, even though the ferrocenium cation is easily reversed back to the original state of Cp2Fe.
Nuclear Magnetic Resonance Spectra
Since the protons on the cyclopentadienyl rings of ferrocene, on the NMR timescale appear to be equivalent then a single peak is expected. As seen in Table 1 a single peak is observed in the NMR spectra of ferrocene at d = 4.1. Ring shielding as well as the effects of the bonding between the iron and the cyclopentadienyl rings can account for the shift. The inductive effect, in an applied magnetic field (Bo), the electrons surrounding the nucleus circulate, setting up a secondary magnetic field opposed to the applied field at the nucleus. As a result nuclei in a region of high electron density experience a field proportionately weaker than those in a region of low electron density, and a higher field has to be applied to bring them into resonance, hence nuclei are shielded. Since the nuclei are in a region of high electron density then the resonance occurs at a relatively high field (low value of d).
Since the cyclopentadienyl rings are bonded onto a iron centre via sigma-donor bonds, [Cp(a1)>Fe(4s, 3pz)], pi-donor bonds [Cp(e1)>Fe(dxz, yz, px, y)] and pi-acceptor bonds [Cp(e2)<Fe(dx2-y2, dxy)].4 The rings will have electron density pulled away via the sigma-donor bonds and pi-donor bonds which would mean that the ring system would become a region of low electron density and hence resonance would occur at a relatively low field (high value of d) but taking into consideration the pi-acceptor bonds on the ring which are the LUMO orbitals, which accept electron density back onto the ring, then the effect of losing electron density is weakened and thus the protons feel a secondary magnetic field and hence are shielded and appear at a low value of d.
Table 1. 1H-NMR Spectrum of Ferrocene.
Name of Compound Shift (d) Structure Type
Ferrocene 4.1 [Fe(C5H5)] Singlet (s)
Nuclear Magnetic Resonance of Nitroferrocene
From Table 1.1 the NMR data shows that there is a singlet at 4.2, which corresponds to the non-substituted ring on ferrocene. However there are two signals of interest at 1.3 and 1.6 both of equal intensity which implies that there are two pairs of protons within different environments to each other due to the nitro group substituent. The information implies that both pairs of protons are directly connected to the carbons of the double bonds since they are being shielded i.e. at a low chemical shift (d) and from the information obtained from the NMR, it implies that two of the four protons are more shielded than the other two since lower chemical shift implies greater shielding, hence more electron density. Therefore the shift at d = 1.3 is implying that the protons, which caused the shift, are next to or in very close proximity to the nitro group, since the nitro group is classed as deactivating i.e. it withdraws electrons from the ring. Thus creating an inequality in the electron density distribution around the ring, which means that the protons furthest away from the nitro group are within an area of less electron density, hence when an magnetic field (Bo) is applied the opposing magnetic field which is caused by the inductive effect is weaker in strength than the one which is induced in the proximity of the nitro group, which leads to the chemicals shift at 1.6, since higher chemical shift implies less shielding.
Table 1.1 1H-NMR Spectrum of Nitroferrocene.
Name of Compound Chemical Shift (d) Structure Type
Nitroferrocene 4.2a (C5H5)Fe(C5H4NO2) Singlet
1.6a (C5H5)Fe(C5H4NO2) Singlet
1.3a (C5H5)Fe(C5H4NO2) Singlet
4.2b (C5H5)Fe(C5H4NO2) Singlet
1.52b (C5H5)Fe(C5H4NO2) Singlet
1.29b (C5H5)Fe(C5H4NO2) Singlet
a) Data obtained from preparation 3
b) Data obtained from preparation 5
From Table 1.2 the data shows that there is a chemical shift at 4.2, which is the singlet of the non-substituted cyclopentadienyl ring. However from the rest of that data obtained from the NMR i.e. chemical shifts d = 0.9, 1.21 and 1.6, they are implying a pseudo-triplet
Since the substituted cylcopentadienyl ring has enhanced acidic hydrogen in close proximity to the nitro group, the hydrogen can be abstracted by the nitro group c.f. Aci formation and umpolung reactions.4, 5, 6 to form a cyclopentadienyl anion which can be stabilised due to the bonding interactions between the ring and the iron centre along with the electron withdrawing effect of the nitro group and the resonance effects of the ring. The proton that is responsible for the chemical shift at 0.9 is in a region of high electron density since the shielding is extremely effective and thus implies that the proton is next to or in very close proximity to the negative charge of the anion. Which implies two possibilities, that the proton is attached to a oxygen atom, since the oxygen atom is very electronegative it can pull the most electron density from the ring via the sigma-bonding network, thus stabilising the negative charge by putting the excess electron density onto itself, thus shielding the proton effectively, further evidence which implies this is that the nitro group has resonance forms and that the overall resonance form is that the negative charge is stabilised across the whole group or that the proton is next to the nitro group where the most electron density is being withdrawn and hence with the added electron density from the negative charge, the shielding is more effective than before. However since they are two other shifts at 1.6 and 1.21, this implies that the latter is not feasible, as the shift at 1.21 is approximately twice as large than the chemical shifts 0.9 and 1.6, which gives rise to the pseudo-triplet in the spectra.
Since the shift at 1.21 is implying that two protons are in an environment where effective shielding is taking place then this also implies that both protons are within the electron density for shielding to take place, where as the shift at 1.6 is implying that no effective shielding is taking place as the shift is higher and therefore in an area of less electron density than the shifts at 1.21 and 0.9. The only point where the electron density would be lower than any other part of the ring system would be directly opposite the nitro group as this is where the effect of the electron density being withdrawn would have the greatest influence over any nuclei which were situated in this position. Hence the positions of the protons implied by the chemical shifts are that one proton is directly opposite the nitro group (H'), since lower effective shielding is taking place, that two protons are joined onto carbons which are joined by a double bond or are in a resonance form so that the protons are feeling the inductive effect (H") and that one proton is attached to the oxygen atom of the nitro group since this proton is the most shielded by electron density and therefore must be in a region of high electron density (H"').
Table 1.2 1H-NMR Spectrum of nitroferrocene
Chemical Shift (d) Structure Type
4.2 (C5H5)Fe(C5H4NO2) Singlet
1.6b (C5H5)Fe(C5H4NO2) Singlet
1.21b (C5H5)Fe(C5H4NO2) Singlet
0.9b (C5H5)Fe(C5H4NO2) Singlet
a) Data obtained from preparation 5.
b) All three shifts form part of a pseudo-triplet even though they are truly singlets.
Infra-Red Spectrum of nitroferrocene
From Table 1.3, the IR spectrum exhibits strong intensity bands at 1454 cm-1 and 1376 cm-1, which implies the aromatic nitro asymmetric and symmetric stretching frequencies respectively.(Appendix I1,I2) However it has been reported7 that the asymmetric stretch for a nitro group attached to a cyclopentadienyl ring is 1500 cm-1 and that the symmetric stretch is 1320 cm-1.7 It has also been speculated that the hydrogens on the ferrocene bend towards the metal centre thus affording better overlap between the p-orbitals of the carbons and the d-orbitals on the iron. Since the nitro group has several resonance forms, then it could be implied that these interact with the d-orbitals on the iron, since there is freedom of rotation around the carbon-nitrogen bond.
If this interaction were being induced then the effect on the wave numbers would be to increase the asymmetric and symmetric stretches. However only the wave number for the symmetric stretch has increased in magnitude while the asymmetric stretch has decreased in magnitude.
This implies that there is an interaction between the nitro group and the d-orbitals, since the nitro group is withdrawing electrons from the ring, which in turn is pulling electrons from the metal centre. This would further imply that the metal centre would benefit from having a interaction with the nitro group since in it's extreme resonance form the nitro group has both oxygens negatively charged and therefore have lone pairs which can interact.
Since an interaction with the d-orbitals would lead to either one or both nitrogen-oxygen bonds to become weaker since electrons are being withdrawn from the bonding orbitals, this would lead to a lowering of the wave numbers that is observed for the symmetric stretch.
However since the asymmetric stretch has decreased in magnitude, this also implies another bonding mode is being induced, since electrons are being withdrawn from the nitrogen-oxygen bonds onto the metal centre. Then in an extreme form of this bonding, it can be implied that one of the oxygens is actually bonded to the metal centre and the nitrogen atom c.f. metallcyclopropane. Therefore when an asymmetric stretch is applied to this extreme resonance form greater energy is needed induce the asymmetric stretch.
Table 1.3 Part of the IR spectrum of nitroferrocene.
Wavenumber (cm-1) Type Structure
1376.04a Symmetric -NO2
1454.80a Asymmetric -NO2
1367.41b Symmetric -NO2
1469.27b Asymmetric -NO2
a) Data obtained from preparation 1
b) Data obtained from preparation 2
Results and Discussion
Solvent effects on the yield of nitroferrocene
As seen in Table 1.4, changing from hexane to THF implies an enhancement of 2.55 in the yield. One of the explanations is that since the 1,1'-dilithioferrocene-TMEDA complex is insoluble in hexane and therefore forms a precipitate on the bottom of the reaction vessel whereas isopropyl nitrate is soluble in hexane. Then the reactants are in a heterogeneous state, thus there is inadequate mixing of the two, which leads to a poor yield since reaction is taking place on the surface of the precipitate. However in THF 1,1'-dilithioferrocene-TMEDA is soluble, along with isopropyl nitrate which means the two reactants are in a homogeneous state i.e. both solvated therefore reaction can take place more efficiently than in hexane as the reaction is not taking place on the surface of the precipitate. However this does not explain why the yield is still poor, one other explanation is steric and electronic effects but this explanation was beyond the scope of this project.
Table 1.4 Effect of solvent on the yield of nitroferrocene
Solvent Amount of ferrocene (g) / mol Amount of isopropyl nitrate (cm3) / (mol) Yield (%) Amount of product (mol)
Hexane(a) 0.93 (0.0049) 1.5 (0.0147) 3.45 0.00017
THF(b) 0.93 (0.0049) 1.5 (0.0147) 8.79 0.00043
a) Data obtained from preparation 1
b) Data obtained from preparation 2
Effect of scaling up the reaction on the yield
From Table 1.5, scaling up the reaction by a factor of 2.5, leads to a yield enhancement of 6.04 which implies that more collisions are taking place and hence reaction between the dilithioferrocene and isopropyl nitrate are occurring despite the electronic/steric effects taking place.
Table 1.5
Solvent Amount of ferrocene (g) / (mol) Amount of isopropyl nitrate (cm3) / (mol) Yield (%) Amount of product (mol)
THF(a) 0.93 (0.0049) 1.5 (0.0147) 8.79 0.00043
THF(b) 2.32 (0.0124) 4.5 (0.0441) 21.54 0.0026
a) Data obtained from preparation 2
b) Data obtained from preparation 3
Results and Discussion
Effect of increasing the isopropyl nitrate on the yield
From Table 1.6, it can be seen that increasing the isopropyl nitrate leads to a yield enhancement of 1.48 at first but then degenerates the yield by a factor of 0.19. This implies that the electronic effects of the isopropyl nitrate i.e. electron-electron interactions are becoming an important factor in the mechanism, since the more isopropyl nitrate that is added, the greater the charge build up in the solvent and therefore the greater the repulsive force that is felt by the dilithioferrocene as it approaches a molecule of isopropyl nitrate, since THF is a polar solvent, then the surrounding solvent molecules do not disperse the charge on the isopropyl nitrate, which would lead to a lowered repulsive force that the dilithioferrocene could easily overcome. However along with the electronic effects, the steric effects will be increased as more molecules, which are present in the solvent would lead to a higher chance of collision and reaction but it would also lead to the chance that the dilithioferrocene and isopropyl nitrate would collide in an unfavourable way, hence not reacting. Since an increase of isopropyl nitrate does lead to a yield enhancement at first, then it is possible to imply that more collisions between isopropyl nitrate and dilithioferrocene take place in a favourable way, thus implying that steric hindrance is low and that the repulsive forces of the negative charge on the oxygen atoms isn't sufficient to cause an energy barrier high enough so that the dilithioferrocene has difficulty overcoming it. Along with this the data implies that for optimum yield with increased amounts of isopropyl nitrate, it is best to use upto 6 cm3 and that anymore would lead to a reduced yield.
Table 1.6 The effects of increasing isopropyl nitrate on the yield of nitroferrocene
Solvent Amount of ferrocene (g) / (mol) Amount of isopropyl nitrate (cm3) / (mol) Yield (%) Amount of product (mol)
THF(a) 0.93 (0.0049) 1.5 (0.0147) 8.79 0.00043
THF(b) 0.93 (0.0049) 6 (0.00588) 12.19 0.00064
THF(c) 0.93 (0.0049) 9 (0.0882) 1.75 0.00008
a) Data obtained from preparation 2
b) Data obtained from preparation 4
c) Data obtained from preparation 5
Problems and Difficulties
As nitroferrocene and ferrocene are both extremely soluble in organic solvents and insoluble in water, along with the added problem that nitroferrocene decomposes in ethanol with sunlight. This means that separation of the two is difficult at best, even on a chromatography column, which leads to the use of sublimation to separate the two finally. Even though this has its own associated problems of risk, that the nitroferrocene decomposes on heating, even though the sublimation is done in vacuo, making the sublimation point lower, hence reducing the risk of decomposition, separation is still not that well defined, hence leads to small impurities in the final product.
The project was a relative success implied by the data obtained. That nitroferrocene was synthesised which was implied by the NMR and IR spectra along with the melting point. As well as the implication that switching from hexane to THF increased the yield, while increased amounts of isopropyl nitrate used lead to a higher yield, though the use of large amounts of isopropyl nitrate lead to reduced yields above the standard isopropyl nitrate amount.
However the IR implied some interaction between the nitro group and the iron centre, thus leading to some form of extra bonding mode. Along with the NMR implying that a hydrogen abstraction by the nitro group from the ring had occurred since a different NMR spectra was obtained from the expected spectrum.
Along with this, problems and difficulties were noted and reported along with possible solutions. Furthermore the project had failed to carry out the synthesis of dinitroferrocene, however this was not unexpected since little or no reports in chemical journals referred to dinitroferrocene, hence reduction to the diaminoferrocene was not possible along with co-ordinating with other metal centres to study the effects and properties.
Since very little is known about nitroferrocene and its applications are either non-existent or speculative at best. Then further study is needed into the structure and interactions between the orbitals on the oxygen atoms and d-orbitals of the iron, if they are any. As well as the applications of nitroferrocene in chemistry today, since some ferrocene derivatives are being used as a chelating ligand for chiral synthesis and catalysis.3
Alternative route to the synthesising nitroferrocene
Since it has been reported7 that direct synthesis of a nitrocyclopentadienyl-transition metal compound is possible via the nitrocyclopentadienide anion and a transition metal halide in THF. Then applying this to the synthesis of nitroferrocene, then it is theoretically possible to synthesise nitroferrocene via an iron halide and the nitrocyclopentadienide anion, which is in excess
It has been reported3 that it is sometimes possible to generate the cyclopentadienyl anion in-situ by an addition of base to a mixture of the iron salt and cyclopentadiene, though applying this to the synthesis of nitroferrocene then it is theoretically possible to synthesise nitroferrocene using this method
.
Along with this further detailed study is needed on the structure of nitroferrocene using X-ray diffraction, IR, NMR to actually find out whether there is any interactions between the d-orbitals and the oxygen lone pairs leading to an extreme resonance form of the oxygen being bonded to the iron centre and the nitrogen atom c.f. metallacyclopropane.
While conducting any experiment, which requires butyllithium, special care must be taken to ensure that no decomposition of the reagent occurs. All the glassware that was involved in the reaction where butyllithium was used were washed with acetone and then dried in oven for five days, this also includes any reaction, which involved the use of dry solvents. Along with this a beaker of butan-2-ol was available at all times while butyllithium was being used, for safety. All the preparations were carried out under a nitrogen atmosphere, via flushing of the equipment to ensure little or no air was present during the preparations. Special care was also taken while handling and disposing of butyllithium and isopropyl nitrate. Ferrocene was purified via recrystallisation from hexane and TMEDA was purified via fractional distillation and then packed under nitrogen. Along with this the THF used was dry to ensure no moisture would be present in the reaction. The butyllithium used in the preparations was at 99.8% purity.
1.1 Preparation of the 1,1'-dilithioferrocene-TMEDA
Into a 3-necked flask, equipped with a nitrogen inlet tube, magnetic stirring bar, pressure equalised dropping funnel and septum, was added TMEDA (1.75 cm3 , 0.0115 mol), hexane (10 cm3 , 0.0750 mol) and n-butyllithium (7.75 cm3 , 0.0820 mol). The mixture was stirred for 10 mins, while a solution of ferrocene (0.93 g , 0.0049 mol) and hexane (40 cm3 , 0.3017 mol) was made up. The solution was injected via septum into the dropping funnel and allowed to run into the reaction mixture over a period of 30 mins. After which the reaction mixture was stirred for another 18 h.
1.2 Preparation of nitroferrocene
The above reaction mixture was cooled to -70oC, then isopropyl nitrate (1.5 cm3 , 0.0147 mol) in hexane (2 cm3 , 0.0150 mol) was added drop wise to the reaction mixture.a The reaction was then left to warm upto room temperature. After which water (20 cm3) and benzene (15 cm3 , 0.1670 mol) was added, the organic layer was separated off and dried over magnesium sulphate. The organic layer was then concentrated in vacuo and chromatographed on neutral alumina (51g), fraction one, a orange/red strip was eluted with benzene-ether (5:1), then fraction two, a orange strip was eluted with benzene-ether (4:1). The solvent from fraction one was removed in vacuo. The crystals were then sublimed in vacuo (60-70oC). A TLC was carried out and one spot was observed. There was insufficient material to carry out a melting point. IR (v , cm-1) 2970.91 , 2833.08 , 1376.04 , 1454.80. The percentage yield was 3.45% (0.04 g , 0.00017 mol).
Footnote:
a) extra isopropyl nitrate (0.5 cm3 , 0.0049 mol) was added via syringe due to a fault in the primary syringe.
b) A 1H-NMR was carried out but due to a technical fault with the NMR, no spectra were obtained.
1. F.A. Cotton, G. Wilkinson, Advanced Inorganic Chemistry, 4th Ed., page 99-105,1163-1164.
2. M. D. Rausch, D. J. Ciappenelli, J. Organomet. Chem., 1967, 10, 127.
3. M. Bochmann, Organometallics 2: Complexes with Transiton Metal-Carbon pi-Bonds, 1st Ed., pages 45-49, 51
4. J. March, Advanced Organic Chemsitry, 4th Ed., page 73
5. W. Carruthers, Some Modern Methods Of Organic Synthesis, 3rd Ed., pages 41-47
6. C. Elschenbroich, A. Salzer, Organometallics, 2nd Ed., pages 314-323
Infra-red spectra of Nitroferrocene
1H-NMR for Nitroferrocene, however the 1H-NMR was having technical difficulties, hence the poor outcome of the NMR
About the Author
Russell Gulliver studied pure Chemistry at the University of Exeter, Devon, UK after leaving his home in a small village near Scunthorpe, North Lincolnshire (South Humberside). His supervisor for the project was a Dr. Osborne, this project was previously published on the X-Chemist website (a website specifically for the Nitroferrocene project which is now defunct).
Russell Gulliver also uses a pen name for writing other books which may include erotica and other interesting subjects. Russell Gulliver does admit that he has some “blond moments” considering he’s a natural blond hair.